SINGAPORE (Reuters) - The WTA Finals can produce all manner of confusing permutations for the players and fans alike as the round robin format reaches a climax, so defending champion Agnieszka Radwanska was relieved to be in a winner-takes-all situation on Friday.

Last year, she won the title with a 1-2 pool phase record, relying on others to help her advance before she battled past Garbine Muguruza in the semi-finals and then Petra Kvitova to register the biggest title win of her career.

On Friday she found herself in a straight shootout situation against big-serving Czech Karolina Pliskova and the Polish world number three rose to the occasion, battling to a 7-5 6-3 victory to set up a last four encounter against Angelique Kerber.

"I think that was simple as it felt like a normal tournament. You win, you're still there. You lose, you're out. Not much maths there," the 27-year-old told reporters.

"But I knew I had to play my best tennis against her. She was showing only good tennis this week as well. Of course, I prepared that she's going to serve good and that's what she did," added the Pole.

"Some of the games I was just pretty much watching her serve."

Looking ahead to Saturday's semi-final against the world number one, Radwanska believes she has a good a chance to emerge victorious.

"She's number one now and she's going to finish the year as number one but I still think all the matches here are 50-50," Radwanska added of her German opponent, who won the Australian and U.S. Opens in a breakout campaign.

"She probably feels more pressure after the season she has had and being number one but I think, well, everyone can really go through."

"Everything is open in semi-finals."

Russia's Svetlana Kuznetsova takes on Slovakia's Dominika Cibulkova in the other semi-final.